Binding reinforcement bars in concrete constructions is known to be a costly operation. By manual processes a wire is curled around the reinforcing bars, and by means of a wire cutter, the free ends of the wire are twisted such that the reinforcing bars are tied together.
Resent considerations not only related to the costs of binding the bars but also related to the working environment, has lead to the development of hand-held, portable devices for binding.
EP 0751270 shows a device for binding reinforcement bars for concrete constructions. The device operates by twisting a wire in a loop by a guide arm. A hook thereby binds the reinforcement bars together by twisting the wire loop.
U.S. Pat. No. 4,252,157 shows a device for binding reinforcement bars, comprising a differential gear for transferring torque from a motor to a binding head and a cutting device, respectively.
EP 1 484 249 discloses a reinforcing bar machine comprising three motors: a feeding motor, a twisting motor and a sliding motor. The feeding motor forms part of a feeding mechanism and is used to feed the wire. A binding wire twisting mechanism includes the twisting motor and the sliding motor.
Further examples of known binding apparatuses are disclosed in U.S. Pat. No. 5,657,799, EP 0 731 238, EP 0 810 153, EP 0 332 532, EP 0 829 596, U.S. Pat. No. 4,362,192, EP 0 751 270, U.S. Pat. No. 4,252,157, and WO 0194206.
It has been found that the ability of the binding apparatus to provide the desired tension in the bound wire is critical for the quality of the binding. If the wire is tensioned too much, the wire may rupture, whereby the user must repeat the binding action hoping that the second binding does also not rupture. If on the other hand the binding is too loose, the binding will most likely not serve its purpose which in many cases is to ensure that two reinforcing bars are forced into contact with each other.
With regard to the twisting of the wire by the binding apparatus, binding apparatuses normally are based on one of two principles. A first in which the wire is twisted as many times as possible e.g. until wire is pulled out of the binding apparatus or until a predetermined torque is reached during the binding process. In a second principle the wire is twisted a predetermined number of times.
One advantage of twisting the wire a predetermined number of times is that the binding time for each binding is held at a minimum. The reason for this is that in the “as many times as possible” process, an excessive amount of wire is often provided in order to ensure that the wire ends are twisted a sufficient number of times so as to ensure a desired strength of the binding. The effect is that the ends must be twisted a large number of times which is time consuming.
However, when the wire is twisted a predetermined number of times, it is difficult to achieve the same tightness of the binding, as the wire path around the reinforcing bare varies from binding position to binding position. In a grid of vertical and horizontal bars, the bars most often will not define a right angle in each intersection—even when this is intended. These small angular variations between intersecting bars make it difficult to provide the same tension in the wire in each binding. The result is that the bindings are either too loose or breaks because they are too tight. Another reason for loose or breaking bindings is that reinforcing bars on their outer surface often are provided with rips/protrusions for mechanically binding the reinforcing bars to the concrete. The ribs/protrusions are spaced apart along the outer surface of the reinforcing bars and depending on the position of the binding relative to the adjacent ribs/protrusions, the wire path may be longer or shorter.
Accordingly, it is an object of an embodiment of the present invention to provide an apparatus that twists the wire a predetermined number of times while it at the same time provides the desired tension in the bindings irrespective of the number of objects to be bound and/or their thickness and/or the number of ribs and/or the position of the ribs relative to the binding.
Moreover, it is an object of an embodiment of the present invention to provide a binding apparatus which reduces the risk of rupture of the wire during binding.
Furthermore, it is an object of an embodiment of the present invention to provide a binding apparatus with which the risk of loose bindings is reduced or even eliminated.